Craig T. Bowman

Mechanism and modeling of nitrogen chemistry in combustion

Our current understanding of the mechanisms and rate parameters for the gas-phase reactions of nitrogen compounds that are applicable to combustion-generated air pollution is discussed and illustrated by comparison of results from detailed kinetics calculations with experimental data. In particular, the mechanisms and rate parameters for thermal and prompt NO formation, for fuel nitrogen conversion, for the Thermal De-NOx and RAPRENOx processes, and for NO2 and N2O formation and removal processes are considered. Sensitivity and rate-of-production analyses are applied in the calculations to determine which elementary reactions are of greatest importance in the nitrogen conversion process. Available information on the rate parameters for these important elementary reactions has been surveyed, and recommendations for the rate coefficients for these reactions are provided. The principal areas of uncertainty in nitrogen reaction mechanisms and rate parameters are outlined.

Evaluated kinetic data for combustion modeling: supplement II

This compilation updates and expands a previous evaluation of kinetic data on elementary, homogeneous, gas phase reactions of neutral species involved in combustion systems [J. Phys. Chem. Ref. Data 21, 411 (1992)]. The work has been carried out under the auspices of the European Community Energy Research and Development Program. Data sheets are presented for some 78 reactions and two tables in which preferred rate parameters are presented for reactions of ethyl, i‐propyl, t‐butyl, and allyl radicals are given. Each data sheet sets our relevant thermodynamic data, experimental kinetic data, references, and recommended rate parameters with their error limits. A table summarizing the recommended rate data is also given. The new reactions fall into two categories: first, to expand the previous compilation relating largely to the combustion in air of methane, ethane and aromatic compounds; and second, provide data for some of the key radicals involved in the combustion of higher alkanes.

Kinetics of pollutant formation and destruction in combustion

Publisher Summary There are five principal classes of pollutant species that are emitted from combustion sources: nitrogen oxides, carbon monoxide, organic compounds, sulfur oxides, and particulates including aerosols. The pollutant emission levels from a particular combustion device depend upon the interaction between physical and chemical processes occurring within the device. In general, the concentrations of various pollutant species in the exhaust differ from calculated equilibrium values, indicating the importance of chemical kinetics in the pollutant formation process. Hence, information on pollutant chemistry is essential for the development of analytical models for pollutant formation and also serves to provide a qualitative understanding of the factors affecting pollutant emissions. This chapter reviews recent investigations of the kinetics of formation and destruction of nitrogen oxides, carbon monoxide, and organic pollutants. It also discusses the coupling between the pollutant chemistry and the combustion process. To predict nitrogen oxide formation rates near the combustion zone, coupling of the nitrogen oxide formation process to the combustion process must be considered. The chapter discusses several approaches for coupling nitrogen oxide formation and combustion.

Control of combustion-generated nitrogen oxide emissions: Technology driven by regulation

Nitrogen oxides in the atmosphere contribute to photochemical smog, to the formation of acid rain precursors, to the destruction of ozone in the stratosphere and to global warming. Over the past 150 years, global emissions of nitrogen oxides into the atmosphere have been increasing steadily. A significant amount of the nitrogen oxide emissions is attributed to combustion of biomass and fossil fuels. Increasingly stringent NOx emissions regulations are being implemented in a number of industrialized countries. These regulations have driven and continue to drive the development of NOx emissions control techniques. This paper reviews existing and some emerging technologies for reduction of NOx emissions from combustion sources and examines the prospects of these technologies for meeting stricter emissions regulations. Both combustion modification and post-combustion methods for NOx reduction are considered. The important role of research on the chemistry of nitrogen oxides in combustion gases in development and optimization of emissions control techniques is described.

An experimental and analytical study of methane oxidation behind shock waves

The oxidation of methane behind reflected shock waves has been studied in the temperature range of 1350° to 1900°K. The mixture compositions studied ranged from 0.2 to 5.0 fraction stoichiometric, and the pressure range was from 1.5 to 4.0 atm. Measurements were made of the pressure, chemiluminescent emissions of OH, CH, Cg, and CO, and the 3067 A absorption of the OH radical during the induction period preceding rapid reaction. The induction times, based on the rapid increase in pressure and OH emission, were found to correlate with initial reactant concentrations of (O2)2.0 (CH4)−0.4. To complement the experiments, an analytical study of methane oxidation was carried out, using a thirteen-step reaction mechanism. The time rate of change of concentrations and thermodynamic properties were calculated by numerically integrating the system of reaction kinetic and state equations. Induction times obtained from the calculations were in good agreement with the experimental results.

Combustor performance enhancement through direct shear layer excitation

Abstract Previous studies of turbulent nonreacting shear flows have shown that flow excitation can provide enhance entrainment and mixing. In the present study, the effects of periodic flow excitation on the performance of a two-dimensional dump combustor were investigated for lean premixed conditions. The flow excitation was in the form of a sinusoidal cross-stream velocity perturbation applied just upstream of the flow separation. The forcing frequencies, chosen such that they corresponded to resonant and off-resonant vortex shedding frequencies indentified in unforced combustion, ranged from 35 to 400 Hz. The effect of forcing on both nonreacting and reacting flowfields was to modulate the formation of vortex structures just downstream of the flow separation. In the nonreacting flowfield, the shear layer spreading rate increased when forcing was applied. In the reacting flow, forcing caused a modulation of the flame structure. Forcing increased the mean CH emission intensity from the flame, which is related to mean volumetric energy release, up to 15%, reduced the rms pressure fluctuation level by up to 30%, and reduced the equivalence ratio at the lean blowoff limit up to 6%. NOx emissions were reduced by up to 20% with forcing. The forcing location and excitation frequency and amplitude are important parameters in gaining effective combustion control. Performance improvements generally increased with increasing excitation amplitude and increasing frequency within the operating constraints of the excitation system. The mean CH emission intensity was found to be proportional to the mean flame surface area, which increased with forcing, suggesting that the observed increase in volumetric energy release was due to an increase in flame area with forcing. The coupling of heat release and the pressure field was investigated using Rayleighs criterion, and the analysis showed decreased flame driving of the dominant low-frequency modes with forcing applied, resulting in a reduction of the magnitude of rms pressure fluctuations.

CO2* Chemiluminescence in Premixed Flames

ABSTRACT Chemiluminescence from species such as CH*, C2*, OH* and CO2* often are used as a quantitative diagnostic in experimental studies of premixed combustion. This paper reports results from a numerical investigation of CO2* chemiluminescence as a quantitative diagnostic in laminar and turbulent premixed flames. Calculations are carried out using a complex reaction mechanism for methane and propane and a model for CO2* chemiluminescence. Relationships between chemiluminescent intensity and both H-atom concentration and heat release rate are quantified as functions of dilution, equivalence ratio, steady and unsteady strain-rate. These relationships are monotonic, but not unique; they depend on which flame parameter is varied. However, the effect of unsteadiness on the relationship for a strained flame is negligible, and this allows the use of chemiluminescence-based diagnostics for measurement of relative H-atom concentration and relative heat release rates in unsteady laminar and turbulent premixed fl...

A shock-tube investigation of the high-temperature oxidation of methanol

The oxidation of methanol behind reflected shock waves has been studied in the temperature range 1545–2180 °K. During reaction, the concentrations of O, OH, H2O and CO were measured using various spectroscopic techniques. Following shock-heating, oxidation appears to proceed through two distinct phases—an induction period, in which the concentrations of radical species and water increase rapidly with little change in temperature, followed by exothermic reaction with the concentrations of radical species and water slowly approaching equilibrium values. The initial stages of reaction were characterized by the time, tmax, between shock-heating and the attainment of the maximum of the product of the CO and O-atom concentrations. For most of the experiments, measured tmax-values were found to correlate with initial methanol and oxygen concentrations and with initial temperature by tmaxC0.5oxygenC0.1methanol = 2.1 ×10−13exp(151.5kJ/RT)sec·(mole/cm3)0.6 To obtain information on the methanol oxidation mechanism, measured tmax-values were compared with results obtained from computer modelling of the reaction. A 19-reaction mechanism is proposed which gives relatively good agreement between calculated and measured concentration profiles. Reactions that significantly influence calculated concentration profiles include thermal decomposition of methanol, attack of the radicals O, OH and H on methanol and thermal decomposition of the intermediate species CH2OH.

A shock tube study of the enthalpy of formation of OH

The standard enthalpy, of formation of the hydroxyl radical (OH) at 298 K, Δ f H 0 298 (OH), has been determined from shock tube measurements spanning the tmeperature range 1964–2718 K and at pressures of 1–2.4 atm. Low-concentration, lean and stoichiometric mixtures of H 2 and O 2 in Ar produce well-controlled leels of OH in a “partial equilibrium” state, with little or no sensitivity to the reaction kinetics. the partial equilibrium OH concentrations are dependent only on the thermochemical parameters of the reacting species, with the heat of formation of OH being the most significant and uncertain parameter. Narrow line width UV laser absorption at 306.7 nm is used to measure OH concentrations with sufficient accuracy (2%–4%) to clearly determine the value of the enthalpy of formation. Over the whole range of experimental conditions, the average determination is Δ f H 0 298 (OH)=8.92±0.16 kcal/mol (37.3±0.67 kJ/mol) with a standard deviation of σ=0.04 kcal/mol (0.17 kJ/mol). This value is 0.40–0.48 kcal/mol (1.7–2.0 kJ/mol) below the previously accepted values, and it agrees with recent theoretical calculations, experimental studies using the positive-ion cycle, and calculations using thermochemical cycles.

The structure of a chemically reacting plane mixing layer

Experiments were performed to examine the structure of a chemically reacting, gas-phase, two-stream plane mixing layer. Temporally and spatially resolved measurements of streamwise velocity and of the concentrations of a reactant and product species and a conserved scalar were recorded across the mixing layer at streamwise locations between Re delta; = 730–2520. Non-reacting flow experiments were conducted to establish the entrainment and mixing characteristics of the layer. Reacting flow experiments were performed using dilute concentrations of the reactants, NO and O 3 , to ensure that the flow field remained isothermal. The probability density functions (p.d.f.s) and associated statistical quantities of the conserved and reactive scalars are compared with results from previous analytical and experimental studies. The data suggest, in concert with the Broadwell-Breidenthal model, that fluid in the mixing layer exists in three states: tongues of unmixed free-stream fluid which, on occasion, stretch across the layer; finite-thickness interfacial diffusion zones of mixed fluid which border the parcels of unmixed fluid; and regions comprising fluid of nearly homogeneous composition. The data also confirm previously reported asymmetry in entrainment rates from the two feed streams and show the important role of molecular diffusion in the mixing process. A fast-chemistry assumption, applied to predict reactive-species concentrations from the measured conserved-scalar p.d.f.s, overestimates the extent of reaction, indicating the importance of finite-rate chemistry for the present conditions. A Damkohler number, based on large-scale mixing times, is shown to be useful in determining the applicability of a fast-chemistry analysis to reacting mixing layers.